This application is a continuation-in-part of copending application Ser. No. 416,911 filed Sept. 13, 1982 now abandoned.
This invention relates to polymeric compositions and processes particularly useful in surface coating compositions, and more particularly pertains to certain reactive self-curing latexes useful as binders in coating compositions.
Water-based coating compositions useful as coatings for metal containers and based on latex polymers are suggested in U.S. Pat. No. 3,991,216. Such polymers are based on interpolymers of copolymerized acrylamide monomer, carboxylic monomer, and other ethylenically unsaturated monomers. However, such polymers are difficult to spray and often exhibit deficient film properties such as lack of resistance to ethanol. Other patents disclosing similar latexes are U.S. Pat. No. 2,978,434 and U.K. Pat. No. 2,034,334, which suggest that premature condensation of emulsion copolymerized alkylol acrylamide derivatives with functional monomers having carboxyl, hydroxyl, or similarly reactive functional groups can be eliminated by copolymerization in two stages. These references teach that first stage monomers contain no alkylol acrylamide monomer and are copolymerized under acidic conditions at 70.degree.-80.degree. C., but second stage monomers containing an alkylol acrylamide together with other functional monomers containing hydroxyl or carboxyl groups are copolymerized under neutral or slightly alkaline conditions at temperatures no greater than 50.degree. C. The disclosed two-stage processes avoid condensation reactions until the coating is baked as a film onto the desired substrate. Similarly, Journal of Coatings Technology, Vol. 51, No. 657 (Oct. 1979) discloses low molecular weight acrylamide latexes adapted to crosslink upon curing as a film.
The present invention discloses a high molecular weight two-stage thermosetting acylic emulsion copolymer containing crosslinked surface polymer chains and carboxyl and methylol acrylamide functionalities particularly useful in the formulation of coatings. The advantage of this invention over similar inventions include improved viscosity stability of the latex/solvent blend due to reduced/controlled particle swelling, improved mechanical stability of the latex resulting in better spray performance of the coating (measured by coverage and blister), and more efficient utilization of expensive self-cure monomers.
It now has been found that certain reactive self-curing latexes containing copolymerized monomers including functional carboxyl, hydroxly, amine or amide monomers in combination with alkylol acrylamide monomers polymerized in a stepwise reaction to concentrate the alkylol acrylamide on the surface of the latex polymer particles provides an improved latex exhibiting surprisingly good rheological properties including spray application. The second stage polymerization takes place at temperatures above about 70.degree. C. wherein a minor amount of alkylol acrylamide reacts with a minor amount of functional monomer during the addition polymerization of ethylenic monomers to produce crosslinked polymer chains in the polymer surface and to provide a relatively rigid or hard polymer particle surface, which appears to stabilize the latex viscosity as well as provide considerable shear resistance during subsequent spray application of the latex. Upon ultimately heat curing of the latex, the latex becomes self-curing by the alkylol acrylamide reacting with the functional monomer groups in the latex. The reactive self-curing latex polymers are high molecular weight polymers containing extremely high molecular weight crosslinked surface polymer chains. The polymer particles advantageously provide good film integrity properties, high solids content, good spray application, and minimal use of solvents. Accordingly, it now has been found that some condensation of functional momoners during the emulsion copolymerization is very advantageous to provide crosslinked polymer chains, viscosity stability, and spray performance without detracting from overall coatings performance.
High pressure spray applications cause severe mechanical shear upon latex particles resulting in very poor spray performance. It is believed that staging the monomer feed in accordance with this invention results in latex particles having a core/shell morphology whereby copolymerization of the monomers in the latter stage under acidic conditions induces a certain degree of surface crosslinking in accordance with this invention. The effect of this crosslinking combined with the steric rigidity imparted by the aromatic vinyl monomer results in a high structured and immobile particle surface. The addition of polymer soluble solvents (e.g., n-butanol) widely employed in water-reducible spray applications have only limited effect on swelling of the latex particle. With this controlled particle swelling, integrity of the latex particles is preserved and viscosity instability associated with long term particle swelling is minimized. Maintenance of latex particle integrity also assists ionic stabilization, which act directly to increase shear stability and thereby improve spray performance. These and other advantages of this invention will become more apparent by referring to the detailed description of the invention and the illustrative examples herein.